Delayed gratification. Hillary Clinton has her eyes on November, but Bernie Sanders says he's in it to win it. And as -- and as he pulls off another primary win, is he pulling Hillary Clinton to the left?

Putin's parade. Russia's president flexes his military muscle and takes some pot shots at the west. What the U.S. and its allies are about to answer by activating of powerful defensive weapon.

And Prince's pills. We're learning more about the pop star's last days amid new searches of his home and new clues that all was not well in the time leading up to his death.

I'm Wolf Blitzer. You're in THE SITUATION ROOM.

ANNOUNCER: This is CNN breaking news.

BLITZER: We're following breaking news. The former GOP nominee, Mitt Romney, drops a bombshell, saying Donald Trump's failure to release his tax returns must mean he's hiding a, quote, "bombshell." Romney says it's disqualifying for a modern -- modern-day nominee to refuse to release his tax returns, warning that could conceal -- and I'm quoting him now -- "conflicts of interest and inappropriate associations."

All that comes as a badly divided GOP is pinning its hopes for party unity on tomorrow's crucial meeting between Trump and the House speaker, Paul Ryan. Ryan is one of many mainstream Republicans who so far have not been able to bring themselves to back Trump. And Trump may be making it even more difficult by making it clear he has no plans to release his tax returns until an IRS audit is complete. The multimillionaire insists there's nothing to learn from his returns.

Also, Russia's Vladimir Putin is cloaking himself in Soviet-style imagery, using a spectacular military parade to take apparent swipes at the United States and its allies, but the U.S. and NATO may be giving Putin something to complain about. They're just hours away from inaugurating a brand-new missile defense system in Europe. I'll speak with Congressman Sean Duffy. Our correspondents, analysts and guests, they will have full coverage of all the day's top stories.

JIM ACOSTA, CNN SENIOR WHITE HOUSE CORRESPONDENT: That's right, Wolf. So far Donald Trump and House Speaker Paul Ryan are saying all the right things ahead of their face-to-face meeting tomorrow here in Washington, but the man who ran for president with Ryan on his ticket four years ago, Mitt Romney, is ratcheting up the pressure on Trump to release his tax returns.

Romney just posted a scathing message on his Facebook page, saying the audit Trump keeps talking about should not stand in his way. And Donald Trump just moments ago tried to clarify the issue with a tweet.

(BEGIN VIDEOTAPE)

ACOSTA (voice-over): One day before what some in Washington have dubbed the Paul Ryan primary and Donald Trump's meetings with top Republicans, the speaker of the House is preaching party unity.

REP. PAUL RYAN (R-WI), SPEAKER OF THE HOUSE: We have to be at full strength so that we can win this election, and that is why we have to go through the actual effort and process of unifying.

ACOSTA: But Trump just gave GOP lawmakers a new reason to worry, telling the Associated Press he will defy campaign tradition and refuse to release his tax returns before the November election, citing an ongoing audit, saying there's nothing to learn from them.

It's a shift from his earlier comments, when he seemed to leave the door open to making his tax information public.

DONALD TRUMP (R), PRESIDENTIAL CANDIDATE: I will absolutely give my return, but I'm being audited now for two or three years, so I can't do it until the audit is finished, obviously.

ACOSTA: But last week, Trump started to dig in his heels, suggesting the IRS could be out to get him.

TRUMP: I say to friends of mine, how often are you audited? Very wealthy people.

BLITZER: Do you think it's political?

TRUMP: I would say yes. I mean, I would say yes.

BLITZER: The IRS is doing this?

TRUMP: I don't know. ACOSTA: Trump has deflected the issue for years. Consider how he

answered the question in 2011 when he was considering a White House run then.

TRUMP: We'll look at that. Maybe I'm going to do the tax returns when Obama does his birth certificate.

ACOSTA: If he sticks to his guns on his taxes, Trump would become the first presidential candidate to fail to release his returns since 1976. Even Richard Nixon released his taxes while he was under an audit.

Mitt Romney, who faced pressure to make his tax information public four years ago, posted a message on his Facebook page, telling Trump to tear down that tax wall. Saying it is disqualifying for a modern- day presidential nominee to refuse to release his tax returns to the voters.

[17:05:00] MITT ROMNEY (R), FORMER PRESIDENTIAL NOMINEE: I predict that there are more bombshells in his tax returns.

ACOSTA: Trump is also standing his ground when it comes to his past comments on John McCain, questioning whether being a captured POW makes you a war hero.

TRUMP (via phone): You know, after I said that, my poll numbers went up seven points.

UNIDENTIFIED MALE: No, I understand all that.

TRUMP: You understand that. I mean, some people liked what I said. But I like John McCain. In my -- in my eyes, John McCain is a hero.

ACOSTA: And Trump's plans to temporarily ban Muslims from entering the U.S.

TRUMP: I'm thinking about setting up a commission, perhaps headed by Rudy Giuliani, to take a very serious look at this problem.

ACOSTA: Just some of the reasons why Republicans like Paul Ryan want to get to Trump better.

RYAN: I don't really know him. I'd rather have a conversation in person than through the media, no offense.

(END VIDEOTAPE)

ACOSTA: Now, just a few moments ago, as we mentioned, Donald Trump posted a tweet on Twitter, trying to clarify this issue. We'll put it up on screen. It says, "In my interview with the Associated Press, I told the A.P. that my taxes are under routine audit, and I would release my tax returns when that audit is complete, not after the election."

So Donald Trump, Wolf, trying to say in that tweet that he was never really saying that he was not going to release his tax returns until after the election. He's saying it's because of this ongoing audit that's underway.

We should point out the Clinton campaign has only begun to seize on this issue. Just today Hillary Clinton said she and former President Bill Clinton have released 33 years' worth of tax returns. The last eight years, she added, are on her website.

We should point out, Wolf, Donald Trump is not legally precluded from releasing his tax returns while they're under audit. He can legally do that right now.

BLITZER: Although he says his lawyers have advised him not to do so. They say that could complicate the audit.

All right. Thanks very much, Jim Acosta, for that. Let's bring in our CNN political reporter, Sara Murray. She's also working the story for us.

Sara, Trump had previously said he was looking forward to releasing his tax returns. When I spoke to him last week, I asked him about his tax returns and he said, "I look forward to doing it, but I'm under audit. I'm going to wait until that audit is complete," citing what his attorneys have told him. What else are you hearing on this?

SARA MURRAY, CNN CORRESPONDENT: Well, Wolf, I think the interesting thing about this and what we saw from Donald Trump tweeting today, you know, he said that he would release his taxes when the audit is done. There is nothing that indicates that the audit will, for sure, be completed before the election, so it's still possible we won't see his tax returns until after the election.

But the other interesting thing is we had asked Donald Trump and his campaign a number of times why they don't release tax returns for prior years, years that aren't being audited. And they have sort of dodged this question. Even though, you know, as you were hearing from Hillary Clinton, she said she's made three decades of tax returns available, Trump could potentially go back five years, three years, let's say, and release returns from those years. And so far his campaign has made no effort to do that.

So I think it's likely he's going to continue to face scrutiny over this issue.

BLITZER: As you know, Sara, he also indicated that in the general election campaign that's now beginning, from his perspective, now that he is the nominee, he will continue to rely on big rallies, which helped him, obviously, a lot over the past year instead of some of the more targeted turnout operations. What are you hearing on that front?

MURRAY: Well, this is sort of vintage Trump, isn't it, Wolf? This is really the strategy that sort of brought him to victory in the primaries was this idea of touching down in every state and getting free media and not paying for polling, not paying for the kind of analytics that we saw other campaign use, for instance, the Cruz campaign or President Obama's campaign. He actually said in this Associated Press interview that Obama's data structure was overrated. Now, I do think we are going to begin to see somewhat of a shift in

the general election. Dana Bash has already been reporting that the Trump campaign is working with the RNC to do a little bit of voter modeling.

And I think the RNC, remember, has a lot of this infrastructure already in place in a number of these battleground states, so that could mean the campaign has to do less of it on their own, but it will be interesting to see if they really do stick with this belief that you can go essentially light or have no analytics and still eke out a victory, Wolf.

BLITZER: All right. So we'll see what happens on that front, as well. Sara Murray, thank you.

Joining us now, Republican Congressman Sean Duffy of Wisconsin. He's a former Marco Rubio supporter. He's been sharply critical of Donald Trump in the past but now says he'll support Trump in the general election. Is that right, Congressman, you are now onboard the Trump train?

And listen, I don't like all that Donald Trump has said, but I know that Donald Trump will be better for the economy and better for our national defense than Hillary Clinton will, so I'm going to work for him and support him.

BLITZER: Do you think he should release his tax returns, even though they're still being audited in recent years?

DUFFY: I side on the side of transparency. So I do think, in terms of tax returns, it is important, though I don't know the specifics of his audit.

All the same, I think that Hillary Clinton should release all of her e-mails and the transcripts of her speeches to big banks and Wall Street. Transparency with all these candidates so American voters know who they are and where they stand on issues is incredibly important on both sides of the aisle.

BLITZER: You saw the Mitt Romney statement that was just posted on Facebook. Among other things, Romney said this. He said, "It is disqualifying for a modern-day presidential nominee to refuse to release tax returns to the voters." He then says, "The potential for hidden, inappropriate situations with foreign entities, criminal organizations or other unsavory groups is simply too great a risk to ignore."

That's a pretty strong statement from Mitt Romney, who has made it clear, by the way, he has no intention of supporting Trump.

DUFFY: I have the utmost respect for Mitt Romney. He's a great guy. But that's -- it's kind of like Nancy Pelosi calling on Hillary Clinton to release her e-mails and the transcripts to Wall Street.

I don't know what he's doing. It seems a little bit like sour grapes. This is indicative of the fact that a lot of Republicans across the country didn't like the tactics that Donald used to win the nomination. He went nuclear, Donald did, on a lot of Republicans that a lot of conservatives have a soft place in their heart for.

And I think there's some residual frustration and anger at Donald. And I think that that's coming through with Mitt Romney's comments, not just today but also in the speech he gave several -- several weeks ago about Donald Trump.

The key to all this, though, as Republicans is you have to recognize that this is your candidate. Donald Trump is your candidate, and you only have a choice between Hillary Clinton or Donald Trump. And you have to decide who will be the best candidate to lead America forward.

And I think for conservatives, the only choice that you have is going to be Donald Trump. And so I think there has to be a healing and a unification.

And Wolf, you talked about this meeting that's going to take place tomorrow here between Paul Ryan and Donald Trump. I'm hopeful that it's going to be a positive meeting. But the good sign is that both of them are on the same page saying they have to get to know each other, No. 1. No. 2, we have to unify the party. No. 3, we have to beat Hillary Clinton. That is a great first step in getting Paul Ryan on board.

And through Paul Ryan I think you get a lot more conservatives, at least in the punditry class, to get on board for Donald Trump.

BLITZER: That's a big issue right now. Congressman, do you understand why Senator Marco Rubio, the man you once supported for the Republican nomination, now says he still doesn't have confidence in Donald Trump as commander in chief of the United States having his fingers on the nuclear codes, but still, because he made a commitment, because he made a promise during one of those debates, he will go ahead and support the Republican nominee.

I guess the question is, how can you support someone that you don't have confidence would be a good commander in chief with his finger on those nuclear codes?

DUFFY: Yes, you know, I -- I didn't understand that analysis either. I think, obviously, Marco Rubio thought that Donald is better than Hillary.

But what I know about Donald Trump is the guy is a winner. Whether he's a winner in business, he's now shown he can win in politics. And I think he's going to do everything he can to win as president. One, keeping us safe, keeping our economy growing, making sure we secure our borders.

And so it's that history that Donald has that gives me more confidence that he is going to be a sound commander in chief and bring more prosperity, you know, and deal with these threats overseas, whether ISIS, Iran, Russia, China. I think that he'll navigate them well, because he likes to win. And dealing with these issues will be a win for Donald Trump and a win for America.

BLITZER: Speaker Ryan, who like you is from Wisconsin, you know him well. He's meeting with Trump tomorrow morning, as we pointed out. How do you bridge the gap, though, between what Speaker Ryan is saying and what Trump has to say?

For example, on issues like immigration, you know, deporting 11 or 12 million undocumented immigrants in the United States, trade, the wall with Mexico, the temporary ban on Muslims coming to the United States. Can you -- can these two guys work those kinds of issues out?

DUFFY: Yes, I think so. So if you look at 30,000 feet, you might see some division on those issues, but if you look a little closer, there is a lot of unity. American -- I'm sorry, Republicans believe that we should secure the border. Paul Ryan believes in that, and so does Donald Trump.

Now, it might be do we -- do we do it with a wall or some other means? I don't think Paul Ryan believes that you can legitimately deport 11 million people, but you should be able to identify who they are and decide whether they get to stay or go back home.

None of us agree that you should make people citizens, but there's a conversation after you secure the border that you talk with whether you give them some form of legal status. So there's a lot of agreement there.

We don't want to see folks from parts of the world that don't like America, that breed radical extremist views, come into America without being vetted. I think there's -- there's unity on that point, but there's not a complete ban on Muslims. I think that Donald Trump has walked that point back.

And on trade, Paul Ryan is a free trader. But Donald Trump also believes in trade. What Donald has been saying, though, is you need to do good trade deals. Good trade is good for America, and I think Paul Ryan would agree with that.

So there's more in common here that these two guys as they meet tomorrow -- maybe it's our Republican beer summit in the Capitol -- can come out closer to being on the same page and unifying the party. And that will be a good thing for, I think, conservatives and Republicans around the country.

BLITZER: They're going to be meeting around 9:30 tomorrow morning. It might be a little bit too early to have a beer, even if you're from Wisconsin. A little bit too early to have a beer.

All right, Congressman, stand by. We're going to talk about the economy, the national debt. Is Donald Trump abandoning some conservative traditional policies? Much more with Congressman Sean Duffy right after this. (COMMERCIAL BREAK)

BLITZER: Our breaking news, the former Republican presidential nominee, Mitt Romney, says Donald Trump's failure to release his tax returns is disqualifying for a modern-day nominee and suggests a hidden bombshell that's overshadowing, at least in part, tomorrow's crucial meeting between Trump and the House speaker, Paul Ryan.

[17:20:15] We're back with Republican Congressman Sean Duffy of Wisconsin.

Congressman, there's also the issue of the debt that seems to distance Donald Trump from traditional Republican positions. As you know, Trump says the U.S. would never default because, as president, he could just have more money printed. How does the Republican Party resolve a statement like that?

DUFFY: You and I might have heard that statement differently. When I heard him say that, I thought he was saying you can't default, because you can print money. I don't know that he was actually saying we should print money or monetize our debt. If he was, I'd give great pushback. We don't want to see a massive policy that increases inflation substantially across the country. That would be a bad thing.

And a lot of us conservatives in the House have pushed back on the Federal Reserve for their quantitative easing, which was basically printing money to buy U.S. debt and mortgage bonds. So I think there was some misunderstanding, a true misunderstanding of what he was saying there and the explanation. What I...

DUFFY: Wolf, what I think is important, though, is there has, in recent weeks, been a lack of clarity on some of the points that Donald Trump has made. And he's had to come back and re-clarify those statements.

And I think that goes to the point that he's not spending enough time with the policy people and more probably with the political people. If you're going to be a successful president, if you're going to get Paul Ryan and the conservative movement on board, you have to understand the policy that is going to make America great again. The ideas that you have to implement through Congress that he'll have to sign.

And so I think he's making mistakes, because he's not put enough time into, you know, getting up to speed on the policy that's going to help us grow the economy and grow our military and keep us strong again. And if he spends more time there, I think that will do wonders for him in his outreach to conservatives.

BLITZER: And I know, I assume you'd love to meet with him, sit down with him and have some good conversations with him, as well.

DUFFY: Yes.

BLITZER: When I spoke to Donald Trump last week at Trump Tower in New York, I had this exchange with him on a subject close to your heart, namely Puerto Rico, and its enormous financial problems right now. Listen to the exchange we had.

(BEGIN VIDEO CLIP)

BLITZER: Should the U.S. bail out Puerto Rico?

TRUMP: No, I don't believe they should. I don't believe they should. And I think, frankly, Puerto Rico is better if they don't, because they'll cut the bonds; they'll cut them way down. They have far too much debt. I will tell you, Puerto Rico has too much debt. So you can't just restructure. You have to use the laws. You have to cut the debt way down and get back to business, because they can't survive with the kind of debt they have.

(END VIDEO CLIP)

BLITZER: What's your reaction? I know you're working on this enormously important issue.

DUFFY: Yes. Donald Trump is spot-on. We don't want to have taxpayer money go to Puerto Rico to bail them out. From Wisconsin, I don't want Wisconsin taxpayers to send my taxpayer money down to Wisconsin [SIC].

So what we're going to do, Wolf, this is a bill that we're going to drop tomorrow. We're going to put in a control board that will partner with the island to get the finances and the budget under control, No. 1.

But No. 2, they have no mechanism for restructuring, so we're going to allow a forum for which the bondholders on the island can negotiate their debt. If they can't come to a joint resolution, they can access the courts and have the courts help them resolve the debt.

But it's true. They have -- they have too much debt. And what you see is when you have massive debt, it creates a lack of investment on the island, which creates poverty. There's 45 percent poverty rates on the island of Puerto Rico. If we don't act right now, that is going to only get worse. More -- more people are going to flee the island and come to the mainland.

And so I'm hopeful that we're going to get a bipartisan bill that comes out of the committee and out of the House that will go to the Senate. I hope neither side plays politics with this, because this is about the real pain that's taking place in Puerto Rico right now.

This is a good bill that both Republicans and Democrats have worked on. Bondholders have seen it; and many of them have bought into what we're doing, because it's fair and it's the right thing to do. So I thought Donald got that one right.

BLITZER: Sean Duffy, the congressman from Wisconsin, thanks very much for joining us.

on all of this, plus a whole lot more. Should Donald Trump release his tax returns now? What can we expect when he meets with the House speaker, Paul Ryan tomorrow morning?

And later, authorities return to Prince's home, this time with a federal search warrant. We have new details on the investigation.

(COMMERCIAL BREAK)

[17:29:00] BLITZER: Our breaking news, Mitt Romney, the former GOP standard bearer, says it's disqualifying for a modern presidential nominee not to release tax returns. And he suggests a hidden bombshell in Donald Trump's failure to do so.

Would it be without precedent -- I know it goes back to the '70s -- for Donald Trump to continue to refuse his release his tax returns?

GLORIA BORGER, CNN CHIEF POLITICAL ANALYST: No, he would be the first nominee since 1976 not to release his taxes. And ironically, a lot of people will think it was ironic that it was Richard Nixon in '73 who first challenged his opponent and said, "You've got to release your taxes. This is important." He was being pressed on a tax issue. And so he released all this is taxes. And since '76, it's actually become something that candidates generally do as a matter of course.

BLITZER: You saw the statement from Mitt Romney.

BORGER: Yes.

[17:30:04] BLITZER: Which is a tough statement. Among other things, hidden inappropriate associations -- raising the possibility -- with foreign entities, criminal organizations, other unsavory groups.

S.E. CUPP, CNN POLITICAL COMMENTATOR: Yes, but this is what you want to know when you're nominating a president. And I'm sure he'd have the same to say about Hillary Clinton and her ties and CGI, the Clinton Global Initiative. This is the kind of stuff you want to know.

BLITZER: But he's raising the issue of criminal -- criminal organizations. What evidence does he have that Donald Trump was involved with criminal organizations?

CUPP: I don't think he has any evidence. I think he's suggesting that it is completely out of bounds and unacceptable for a person running to be the president of the United States to keep that information hidden.

And what I'm hidden, inexplicably from Trump supporters, is that it's none of our business; and that is truly a truly terrifying and very undemocratic defense, I think.

BLITZER: Mark, is there any way Trump can be literally forced to release those tax returns, even if the IRS is still continuing those audits?

MARK PRESTON, CNN POLITICS EXECUTIVE EDITOR: Well, Wolf, in a simple answer, no. He's an unconventional candidate that has won or is on his way to winning this Republican nomination without any help of the Republican establishment. There's nobody that seems to be able to convince Donald Trump that he needs to run his campaign a certain way or that he needs to do certain things or he needs to be careful about what he says. He really does march to the beat of his own drummer, and I think on this tax issue, he's going to continue to do so until he decides that he wants to release this data.

BLITZER: He has shown some flexibility, Rebecca, on certain issues in recent days since he got, for all practical purposes, the nomination. Does that concern Republicans on some of these core issues? He's sort of moving around?

REBECCA BERG, REAL CLEAR POLITICS: It absolutely concerns Republicans, Wolf. And flexibility, I think, is the generous term from what we've seen from Donald Trump. He's been wildly inconsistent on many basically core policy issues for Republicans, including today.

One of his top policy advisers, Sam Clovis, said that he could see Trump's administration possibly tackling entitlement reforms once he's in the White House, and Trump has said he's not touching entitlement. So more inconsistencies from his campaign now.

It just really shows how malleable he is on policy, how this isn't really a focus of his campaign. He's wanting this to be about style, less about substance. I wonder if he's going to be able to maintain that for the whole campaign.

BLITZER: Trump says and he made it clear to me, you know, these are negotiations he's going into. He has his opening bargaining position. He's going to go in, meet with the Democrats on the Hill, meet with others; and then eventually, they'll come up with a compromise, because he says that's the way it should be done.

CUPP: But who are we electing? Who are we nominating?

BORGER: You know, malleable is one thing. Negotiating is one thing. But having a set of beliefs is something else. And I think what concerns Republicans like S.E. -- I don't want to put words in your mouth.

CUPP: Go ahead.

BORGER: ... is this question of how conservative is he? What does he really stand for? And who is he? And, you know, to bring this back to the tax issue, I think that we

have all gotten used to a certain transparency, whether it's on the issues, "This is what I believe. This is what I would do if I became president," not "I would negotiate," but "this is what I believe in."

As well as on things like your taxes, which tell you a lot about a person, about potential conflicts of interest, about where you give charitable money, where your money has come from. And you know, we've grown accustomed to transparency in our -- in our electoral process.

CUPP: And we generally ask for more of it of administrations. And, you know, the current administration has faced criticism for its opacity. The prior administration faced criticism for its opacity. So this idea that some of this is none of our business is kind of frightening.

BLITZER: I guess, Mark, the key question is the big meeting tomorrow between Donald Trump and the House speaker, Paul Ryan. What's your analysis? Are they going to come out with their hands together or is there still going to be a problem, the speaker still refusing to endorse him?

PRESTON: I think they're going to come out and they're not going to be holding hands, but they will have made progress to getting to the end goal of Speaker Ryan and other Republicans getting behind the Donald Trump candidacy.

Here is the issue right now for Republicans. OK, even if they don't like Donald Trump, he's still going to be at the top of the ticket. If Donald Trump goes into the November election, does not get any help from the Republican Party, gets slaughtered, OK, gets slaughtered, that is going to have ramifications down ballot.

There's a good chance then Republicans could lose control of the Senate. There's a good chance they are going to lose a substantial number of seats in the House of Representatives. So even though they don't like Donald Trump, there's going to have to be some coming to Jesus moment for them, Wolf, to try to protect their own interests in Congress.

BLITZER: You think they're going to come out working together or still split?

CUPP: Well, I know that Paul Ryan really wants to get on board. He wants to help Donald Trump, help him to get on board, so that's an earnest overture. But I would just caution, you know, after every time that Donald trump slammed the RNC and then went into a meeting with Reince and then came out to said everything was great, he went right back to slamming the RNC. So I think any detente achieved is probably temporary.

[17:35:24] BORGER: I think that Paul Ryan's concern is really his caucus. And I think he not only has exhibited a certain amount of candor, saying, "I'm not there yet," but I also think he's given his caucus a place to go. Some members, if they don't feel comfortable endorsing Trump, there's the Ryan agenda that he has provided. But I've heard from some members who say, you know, "I don't see how

he gets to yes and ends this." I think they have to do something together, because as you point out, he is the -- he is the nominee of the party.

BLITZER: It may take more than one meeting. We'll see what happens tomorrow morning.

BORGER: Well, conversations would be good.

BLITZER: Conversations are good, always good. All right, guys, stand by. Coming up, what's behind Vladimir Putin's latest show of military force and how a new move by NATO could push tensions even higher.

(COMMERCIAL BREAK)

[18:40:54] BLITZER: In the Democratic presidential race, Hillary Clinton isn't talking about her latest loss to Senator Bernie Sanders in the West Virginia primary. Instead, she's looking ahead to a general election matchup against Donald Trump.

Our senior Washington correspondent, Joe Johns, is following the Democrats for us. Joe, Hillary Clinton also trying for a big finish to this primary season.

JOE JOHNS, CNN SENIOR WASHINGTON CORRESPONDENT: That's certainly true, Wolf. After his big win in West Virginia, Bernie Sanders was not only running hard today; his campaign was using some attention- grabbing tactics to describe the race.

A fund-raising letter calling polls that show Hillary Clinton running neck and neck with Donald Trump in battleground states as scary and terrifying and suggesting a Clinton nomination would be courting disaster to protect the status quo.

(BEGIN VIDEOTAPE)

JOHNS (voice-over) Hillary Clinton is still waging a primary fight.

HILLARY CLINTON (D), PRESIDENTIAL CANDIDATE: I believe if we do what we need to do in the next few weeks, we will have a big victory on June 7 that will take us all the way to the White House.

JOHNS: But these days it's her likely general election opponent that's getting most of her attention.

CLINTON: I believe with all my heart that we are better than the campaign that the presumptive nominee for the Republican Party is running.

JOHNS: That as Bernie Sanders vows to stay in the race, touting a 15- point win in Tuesday's West Virginia primary.

SEN. BERNIE SANDERS (D-VT), PRESIDENTIAL CANDIDATE: And let me be as clear as I can be. We are in this campaign to win the Democratic nomination. JOHNS: But he acknowledges an uphill climb in the delegate count,

with Clinton now just 148 delegates shy of clinching the nomination.

SANDERS: I say to those super delegates in the states where we won landslide victories, listen to the people of your state.

JOHNS: And Sanders is not backing off his attacks, criticizing Clinton for taking donations from the family that owns Wal-Mart.

SANDERS: So I say to Alice Walton, maybe instead of donating $300,000 to Secretary Clinton, pay your workers a wage that they can live on.

JOHNS: As the Democratic contest carries on, Sanders is pulling Clinton to the left on a range of issues, including trade, immigration and the minimum wage. The most recent case, health care, with Clinton putting fresh emphasis on a proposal to expand Medicare coverage by allowing people to buy in early.

CLINTON: I'm also in favor of what's called the public option, so that people can buy into, you know, Medicare above a certain age, which will then take a lot of the pressure off the costs.

JOHNS: Clinton is also facing increasing attacks from the Republican side.

JOHNS: Clinton firing back at Trump today for his refusal to release his tax returns.

CLINTON: My husband and I have released 33 years of tax returns. We've got eight years on our website right now. So you've got to ask yourself, why doesn't he want to release them?

(END VIDEOTAPE)

JOHNS: Meanwhile today, Vice President Biden said on ABC he's confident Hillary Clinton will be the nominee.

And House Democratic leader Nancy Pelosi said she wasn't ready to endorse anyone, but that the next president, whoever she may be, will be one of the most qualified people to ever enter the Oval Office, Wolf.

BLITZER: That was a quote from her when she used the word "she." All right. Thanks very much, Joe, for that.

Coming up, new muscle flexing by Vladimir Putin's military. Will new moves by NATO provoke him even more?

Also, investigators return to Prince's home, this time with a federal search warrant. We have new details about what they're looking for.

[17:45:00]

(COMMERCIAL BREAK)

BLITZER: Russia's Vladimir Putin is cloaking himself an imagery from Soviet days using a spectacular military parade to take some fresh shots at the United States and its allies.

Our Brian Todd has been looking into all of this. So, Brian, what are you learning?

BRIAN TODD, CNN CORRESPONDENT: Wolf, tonight tensions between Putin's government and the United States are at a boil. We're told that in just a few hours a new missile defense system installed by the U.S. and its European allies is going to be activated in Romania. Now that is certain to anger Vladimir Putin, so more military provocations could be coming from the Russians who have just shown us a massive display of their fire power.

(BEGIN VIDEOTAPE)

BRIAN TODD, CNN CORRESPONDENT: Vladimir Putin in his element.

[17:50:02] Victory Day in Moscow, commemorating the Soviet Union's World War II role in the defeat of Nazi Germany. But this is also the Russian president's moment to show off his new military, the one he spent billions to overhaul.

Putin eager to display the discipline of his forces, soldiers, high staff in unions. Tilt their heads in deference and for the first time a battalion of female soldiers appears marching in white skirts.

WILLIAM POMERANZ, THE WOODROW WILSON CENTER: He is trying to project strength and he's trying to harken back to the time when the country was united in a common purpose to fight a common enemy. And that is the symbol of World War II and that's what he's relying on. And again, for many Russians who were raised during the Soviet period, this is what they understand.

ANGELA STENT, TRANSATLANTIC ACADEMY: Putin is trying to appeal to all of the positive elements of Russia's past, both imperial and certainly Soviet.

TODD: Projections of power and masculinity are Putin's political life blood. He loves being shown playing exhibition hockey, though he recently took a tumble on the ice. Putin seemingly couldn't resist one salvo aimed at this rivals.

PRES. VLADIMIR PUTIN, RUSSIA (Through Translator): Double standards as well as short-sighted indulgence of those who nurture criminal plans are impermissible.

TODD: A remark interpreted by some as Putin taking a swipe at the U.S. for its support of some rebel groups in Syria which Russia considers terrorists. This comes in the wake of several close calls which U.S. officials view as dangerous. A Russian jet barrel-rolling a U.S. military aircraft. And this Russian buzzing of the USS Donald Cook moves seen as showing Putin's anger over the U.S. ramping up its forces and maneuvers in Eastern Europe.

On Thursday, NATO inaugurates land based missile defense system in Romania which Russia is protesting. But U.S. and NATO officials say those deployments are defensive, a response to Putin's aggression in Crimea and Ukraine.

STENT: This is definitely a very dangerous moment in the relationship between the United States and Russia. President Putin and his colleagues have used rhetoric about the possibilities of limited nuclear war. President Putin has said in response to some of the things emanating from the White House, you know, maybe the United States has forgotten that we're a nuclear power.

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TODD: And experts say look for both sides to ratchet up the tension even more. Putin is likely to send troops and possibly even nuclear warheads right to NATO's doorstep.

Interesting tonight, Wolf, in that Victory Day parade, Putin also unveiled this new very mysterious unit the military called the National Guard. It is designed to protect him in the event of a coup attempt.

BLITZER: Brian Todd reporting for us. Thank you, Brian.

Now we're also right now getting some new details about the investigation into what led to Prince's unexpected death last month.

Our justice reporter Evan Perez has been working his sources.

Evan, what are you learning about the doctor who treated Prince in the days leading up to his death?

EVAN PEREZ, CNN JUSTICE CORRESPONDENT: Well, Wolf, we're talking about Dr. Michael Schulenberg. He's a doctor who saw Prince at least twice in the weeks leading up to his death and was at the compound, at the Paisley Park compound, on the day of his death. Now the DEA and U.S. authorities who are assisting in this investigation with local authorities, they want to know more about the relationship between Dr. Schulenberg and members of Prince's inner circle as well as his relationship with Prince.

Now we're are told that, you know, they're doing more investigation to figure out where these pills came from. We are not saying that the Dr. Schulenberg is the source of those pills. As a matter of fact, his attorney, Amy Connor, sent us a statement saying that Dr. Schulenberg is cooperating with investigators. At this point, Wolf, investigators have not found any indication that

Prince had a valid prescription for these opioid pills that he had in his home when he was found dead. And they've -- are telling us that they don't believe that this is a case of doctor shopping. Instead what they believe is the case that perhaps these drug runners that we've talked about on this show, people who obtain these medications illegally, that perhaps those people were responsible for providing these pills to Prince.

Again, they're still waiting for any indication that these pills played a role in his death. We're waiting for toxicology.

BLITZER: But is all this investigation leading to some sort of criminal case?

PEREZ: Well, increasingly the DA is bringing more of these cases. It used to be that the federal government didn't do these types of cases, but we do believe that that's what they're pursuing very visibly here in this case -- Wolf.

BLITZER: Evan Perez reporting for us. Evan, thank you very much.

The hidden epidemic of prescription drug abuse and how to stop it are the subject of a special "ANDERSON COOPER 360" town hall, "Prescription Addiction, Made in the USA."